There have been proposed objective lenses for reading both a DVD comprising a transparent substrate having a thickness of 0.6 mm and a CD comprising a transparent substrate having a thickness of 1.2 mm (JP-A-10-255305, JP-A-11-16190, JP-A-11-2759).
In these conventional techniques, a phase shifter having a concave portion or a convex portion, which is formed in an annular belt-like shape around the optical axis as the center, is provided in a surface of the objective lens having an aspheric surface. The basic structure of the objective lens is optimized so as to be capable of reading a DVD, preferably. When a CD is to be read, the phase is corrected by the phase shifter so that the aberration is made as small as possible. As a result, for both DVD and CD, the correction can preferably be achieved with respect to the on-axial aberration, in particular, the on-axial spherical aberration.
In the conventional techniques, however, a preferred correction could not be achieved with respect to the off-axial coma aberration at the time of reading a CD. Namely, since the off-axial coma aberration was large, the optical performance showed a large reduction when the light source, the objective lens, etc. were inclined from the optical axis or shifted from the optical axis. Therefore, high accuracy was required in positioning the objective lens, etc. in assembling the optical device. Hence, productivity was poor.
Further, in the requirement of high accuracy in the determination of the position of the objective lens, if a mechanism for moving the lens or the light source (a moving mechanism) was worn out, the objective lens was inclined or shifted from the optical axis, whereby there caused gradual deterioration of the optical performance with a lapse of time.
Further, since the off-axial coma aberration was large, an allowable range to the positional determination of the objective lens, in particular, a deviation of the axis in autofocus driving to the objective lens became narrow when the optical device was actually operated. Therefore, there was the problem that the optical performance was reduced.
An object of the present invention is to solve the above-mentioned disadvantages, and to provide an, objective lens capable of correcting well both the on-axial aberration and the off-axial aberration when plural kinds of optical disks such as, for example, DVD, CD, etc. are recorded or read, and to provide an optical device using the objective lens.